This invention relates to an improved compound bow of the type utilizing coacting spring member and bow limbs and, in particular, to an improved compound bow having programing means for regulating the pull weight of the instrument whereby maximum pull weight is attainable at an intermediate draw position.
Allen in U.S. Pat. No. 3,486,495 discloses a compound bow having a draw control mechanism associated therewith for regulating the pull weight over the draw length of the bow. The mechanism involves a pair of cams that are suspended from the tips of the bow limbs and about which the bow string is trained. In operation, the cams act between the bow limbs and the bow string to control the relative motion therebetween as the string is drawn. The cams are contoured to vary the pull weight of the bow so that maximum pull is reached at an intermediate draw position. As the bow string continues to move back from the intermediate position, the pull weight drops off so that the archer is not required to sustain the bow at maximum pull weight while he is aiming. This leads to greater accuracy as well as a smoother arrow release. Allen also points out that the use of a variable pull weight increases the release energy of the bow when compared to a similar bow which releases at maximum draw.
Hofmeister in U.S. Pat. No. 3,854,467 and Trotter in U.S. Pat. No. 3,923,035 describe variations of the Allen type bow wherein the camming device is suspended from the bow riser rather than the limb tips. In both designs, however, a pulley is mounted in the limb tip and the bow string is trained over the pulley and then brought around the cam to impart the desired motion to the string. An extremely complex and exposed bow string network is thus created which has a number of crossover runs. The crisscrossing bow line makes handling of the device difficult. The exposed, closely positioned runs can also become easily entangled or snared in foreign objects thereby limiting the bow's usefulness, particularly in rough terrain. More importantly, hanging heavy pulleys and cams on the outer extremities of the limbs creates unwanted, and generally unpredictable, inertial forces in the bow system that adversely affect the performance characteristics of the bow.
Islas in U.S. Pat. No. 3,981,290 discloses a compound bow that preserves all the advantages of the Allen type bow while at the same time avoiding the disadvantages associated therewith. Primarily, the Islas design eliminates the need for pulleys and cams to be hung upon the limb tips and provides for a cleaner bow configuration. In the Islas bow, a pair of elongated spring members are extended outwardly from the top and bottom sections of the bow riser. The spring member act in concert with companion bow limbs to deliver the desired pull weight. In assembly, the base of each limb is adapted to ride along the working surface of a stationary cam affixed to the riser. The midsection of the limb is pivotably mounted in the free end of the adjacent spring member and the bow string is simply and cleanly connected to the tips of the limbs. As the string is drawn, the limb is caused to ride along the profiled cam surface thereby imparting a pre-programed response to the bow over the length of the draw.
Although the Islas bow has demonstrated itself to be a highly effective instrument, adjusting and maintaining the stationary cams in alignment with the movable bow limbs sometimes proves difficult. Also, in the Islas bow, the bow limbs are arranged to be pulled back against the resisting spring members which may on occasion create nonsymmetrical friction forces acting on the limbs which can adversely affect the performance of the bow.